Patent document 1 describes the method for foam injection molding comprising firstly a step of a short-shot injection (leaving a void) of injection material composed of a resin and an expanding agent into the mold cavity with an installed insert, and secondly a step of filling the void by the expanding action due to foaming of expanding agent, wherein the resin is composed of a base resin and a low-molecular-weight resin which is of the same type as the base resin but has a molecular weight lower than that of the base resin.